The invention relates to electrically switchable particle devices employing a system of flakes or platelets of either polymer liquid crystals (PLC) or birefringent polymers (BP) suspended in a fluid host medium. The devices in accordance with the invention are useful in information display, optics, and photonics, including but not limited to, reflective or transmissive information displays capable of color switching and as flexible media for information display applications on either flat or curved surfaces (e.g. large-area signs, automobile dashboards, heads-up displays, and xe2x80x9celectronic paperxe2x80x9d. The invention also provides improved switchable and tunable devices for color manipulation (i.e. switchable or tunable color filters), switchable and tunable optical retardation or modulation elements for polarized light at desired wavelengths or bandwidth, switchable micropolarizers, switchable xe2x80x9csmart windowsxe2x80x9d for either energy or privacy control, switchable conformal coatings for use in decorative applications, and switchable coatings for applications in military security, camouflage, substrate reflectance control, document security, anti-counterfeiting, and object tagging and identification.
Considerable research effort has been focused on technologies for information display such as liquid crystal devices, field-emissive devices, plasma devices, and most recently, particle-based image displays. Particle-based displays, initially investigated over 30-40 years ago, rely on the motion of particles suspended in a fluid host medium. The particles respond to an applied electric or magnetic field to produce either a change in reflectivity or color of incident reflected light or to modulate or change the polarization state of transmitted light. Such effects are induced by either translation or rotation of the suspended particles and are based on physical phenomenon such as magnetic polarization, electrophoresis, dielectric polarization or other forms of current-induced effects. C. R. Tate, U.S. Pat. No. 3,406,363 issued Oct. 15, 1968 describes multicolored xe2x80x9cmicromagnetsxe2x80x9d of varying magnetic strength made of materials such as barium ferrite, in a liquid suspending medium, and the manipulation thereof by an external magnetic force to produce color displays. W. E. Haas et al, U.S. Pat. No. 4,076,387, issued Feb. 28, 1978 describes metal flakes, such as aluminum, of  less than 325 mesh size (xcx9c45 xcexcm) dispersed in either a water-based or a hydrocarbon-based ferrofluid to construct a reflective display device switched by an electromagnet for use in ambient lighting conditions. In addition, I. Ota, U.S. Pat. No. 3,668,106, issued Jun. 6, 1972, describes an electrophoretic display or recording device based on charged particles manipulated by an electric field that altered the device reflectivity.
C. W. Jacob, U.S. Pat. No. 3,967,265, issued Jun. 29, 1976, describes a light gating display consisting of small conductive particles (aluminum or graphite) dispersed in a continuously circulated dielectric fluid. The device of the Jacob patent is addressed indirectly using an electric field generated by selective illumination of an integral photoconductor, and functions as an image relay or image converter, whether in reflection or transmission. Jacobs mentions that the particle dispersion can be replaced by a pure, particle-free liquid crystal fluid host to produce a device that functions in a similar manner.
U.S. Pat. No. 5,650,872, issued to R. L. Saxe et al. on Jul. 22, 1997, describes a xe2x80x9cSuspended Particle Devicexe2x80x9d (SPD) that relies on sub-micron, anisometric, light-polarizing particles dispersed in a host medium for its function. Application of an electric field causes the particles to reorient. Two-color switching is possible, but very large cell path lengths (125-825 xcexcm) and high drive voltages ( greater than 2000V) are required, and the contrast difference between on and off states is low.
More recently, particle-based devices employing microencapsulation techniques have come to the forefront due to great interest in their use as new forms of flexible, re-writeable, re-usable electronic storage media that could take the place of newspapers or other printed material. Such paper-like information storage media that could be updated electronically from the Internet or other source of stored information is projected to have a large economic impact due to the wide scope of potential applications and market areas that such a device could address. These devices consist of a particle/fluid host dispersion that itself is microencapsulated in a polymeric, film-forming binder material to form a flexible, electrically-addressable sheet with bistable switching characteristics that can retain text and graphics images for prolonged periods of time.
In this connection, N. K. Sheridon et al. in U.S. Pat. Nos. 4,143,103, 4,126,854, 5,344,594, 5,389,945, 5,708,525, 5,739,801, 5,751,268, 5,760,761, 5,767,826, J. M. Crowley in U.S. Pat. Nos. 5,754,332 and 5,825,529, D. K. Biegelsen et al. in U.S. Pat. No. 5,717,283 and J. D. Mackinlay et al. in U.S. Pat. No. 5,737,115, describe various forms of a xe2x80x9cGyriconxe2x80x9d device consisting of 50-500 xcexcm xe2x80x9cbi-chromalxe2x80x9d glass or polymeric spheres contained within fluid-filled cavities that are themselves encapsulated in a flexible polymer matrix. Each sphere is formed such that its opposing hemispheres are of different colors, and can be made to rotate from 90-180xc2x0 in the presence of an electrical field applied across the film because the two hemispheres bear opposing surface charges. Drive voltages increase with increasing diameter of the rotating spheres and the cavities containing them, and produce bistable switching. The Gyricon device functions in reflective mode.
Another microencapsulated particle display technology is E-Ink(trademark), described in U.S. Pat. No. 5,961,804 issued to J. Jacobson et al. and by J. D. Albert et al, U.S. Pat. No. 6,017,584. This technology is based on an encapsulated xe2x80x9celectrophoretic dispersionxe2x80x9d of negatively and positively charged microparticles ( less than 1 xcexcm in diameter) in an isotropic, dielectric host fluid that in turn is encapsulated in a polymer binder. According to Jacobsen and Albert, microencapsulation of the electrophoretic dispersion is an essential element for avoiding the particle agglomeration and sedimentation that commonly occurs in conventional electrophoretic displays. Unlike the Gyricon device, the E-Ink device functions by translation of particles towards the top or bottom of the microcapsule, depending on the sign of the applied voltage. The particles are generally composed of inorganic or metallic particles that are coated with a polymeric coating. Other pigments, metallic flakes or retroreflectors are added to the electrophoretic dispersions either to produce switching between colors or to producing changes in reflectivity, respectively. A nematic liquid crystal fluid can be used in place of the isotropic, dielectric host fluid in the dispersion to modify the switching threshold and bistability of the device. A. Somlyody et al, U.S. Pat. No. 4,305,807 issued Dec. 15, 1981, reports the use of a nematic liquid crystal as a fluid host for charged-particle displays to provide a threshold response characteristic in a similar manner as the devices of the Jacobsen and Albert patents.
Although many examples abound in both the patent and open literature of particle-based devices employing inorganic or metallic particles, instances of reported electro-optical particle devices that employ birefringent polymers or polymeric liquid crystals as the active particles are nearly non-existent. One example is the use of in-situ polymerized nematic liquid crystal spheres prepared by polymerizing a UV-curable liquid crystal reactive monomer dispersed in glycerol in the presence of an appropriate photoinitiator as reported by D. Cairns et al. in the Society for Information Display (SID) Digest of Technical Papers, XXX, p725-728 (1999). This device differs substantially from those described in the patents described above in that (1) the resultant polymerized nematic liquid crystal spheres are themselves birefringent and bipolar, and (2) these spheres can be reoriented by the coupling of an applied AC electric field to the dielectric anisotropy of the polymerized LC material. Thus, Cairns et al proposes an Electro-Mechano-Optical (EIMO) device which functions similarly to the Gyricon device in that the optical effect is produced by field-induced rotation of spherical particles suspended in a fluid. But unlike the Gyricon device, which functions in reflection, the EIMO device relies on a difference in nematic birefringence produced by the rotation of the polymerized nematic LC spheres to produce the optical effect, and thus operates as a polarization rotation device in transmission only.
In S. Faris et al, U.S. Pat. No. 5,34,557, issued Nov. 15, 1994, particles, or xe2x80x9cflakesxe2x80x9d, generated by thermal fracturing of polymer cholesteric liquid crystal (PCLC) polysiloxane films which are aligned in the Grandjean texture are disclosed. These flakes range in size from hundreds of microns to less than a micron, and exhibit optical properties identical to the parent films from which they were derived. Because they originate from an aligned PCLC film that has a relatively high glass transition temperature (Tg), their unique optical properties (selective reflection wavelength, birefringence) and alignment quality are xe2x80x9cfrozen-inxe2x80x9d and are thus essentially insensitive to thermal or mechanical disruptions. These unique optical properties can be transferred to other polymer xe2x80x9chostsxe2x80x9d by dispersing the flakes into a solution of the polymer and using it as a binder to prepare passive (i.e. non-switchable) conformal coatings with unusual optical properties for applications ranging from document security to exterior coatings for motor vehicles. Other methods of generating PLC flakes and pigments have also been reported by I. A. Shanks et al.. in U.K. Patent 2,276,883, C. Muller-Rees et al. in U.S. Pat. No. 5,851,604, R. W. Phillips in U.S. Pat. No. 5,279,657 and W. Hou et al. in U.S. Pat. No. 5,587,242. Very recently, Faris et al have also reported a method for preparing patterned platelets based on photocrosslinkable PCLC materials.
The hue, saturation and brightness of PCLC films have all been extensively studied. The color properties and chemical compatibility of PCLC flakes in some host fluids were first studied by E. Korenic et al., as reported in Mol. Cryst. Liq. Cryst. 317, 197-219 (1998). Two-color superimposed coatings have an additive color mixture, which differs from the subtractive color effects obtained with conventional inks, dyes or pigments. Because the colors are highly saturated and show a dependence on the angles of illumination and observation, their visual appearance has been used as the basis for art or the manufacture of commercial products such as paints and cosmetics. L. Li, in U.S. Pat. No. 5,691,789, proposes that specialized curing cycles for thermally or photochemically cross-linkable PCLCs can be used to induce a pitch gradient into the PCLC film. By controlling the gradient of the pitch, films with broad reflectance bands ranging from the visible to the near IR can be fabricated. One application of these materials is as broadband, non-absorptive polarizers in LC displays.
It has been discovered in accordance with the invention that when flakes or platlets of polymer cholesteric liquid crystals (PCLC) or other birefringent polymer (BP) materials are dispersed or suspended in a fluid host to provide a flake/host system, they are suprisingly sensitive to an applied electric field and are readily switchable to different angular orientations. As will be apparent from the above discussion of the prior patents and publications, the sensitivity of those flakes or platelets when dispersed or suspended in a fluid host has not heretofore been appreciated. These flakes or platelets are referred to generically as electrooptically sensitive flakes hereinafter. The performance of devices according to the invention is dependent on both the character of the electrooptically sensitive flakes and the host, which preferably have the following characteristics,
1) The flake material is chemically compatible with the host medium (i.e., completely insoluble);
2) The density of both the flake material and the host medium is as closely matched as possible to aid in keeping the flakes uniformly suspended in the host and prevent them from agglomerating, settling to the bottom, or rising to the top of the device;
3) The host fluid has a high a resistivity (xe2x89xa7109 xcexa9-cm) sufficient to eliminate competing electrohydrodynamic effects;
4) The flakes are within the range of 20-40 xcexcm wide with a length-to-width ratio of 2-3 to observe optimal switching and contrast;
5) The viscosity of the host material is the lowest value as will maintain the flakes in suspension to minimize response time;
6) The index of refraction of the flake material and the host fluid is closely matched as possible to avoid scattering losses.